The present invention relates to ink jet printing and, more particularly, to a technique for reducing ink usage on the printed page while maintaining image quality.
Ink jet printing systems are known in which a print head defines one or more rows of orifices which receive an electrically conductive recording fluid, such as for instance a water base ink, from a pressurized fluid supply manifold and eject the fluid in rows of parallel streams. Printers using such print heads accomplish graphic reproduction by selectively charging and deflecting the drops in each of the streams and depositing at least some of the drops on a print receiving medium, while others of the drops strike a drop catcher device.
When applying ink jet droplets to a print substrate, excessive ink can create an overly dark image and a loss in detail in the shadow areas. The image quality is dependent on not only the amount of ink being placed on the substrate, but also on the porosity of the print substrate. The combination of printing many or large ink drops on a printing surface that is not very porous can cause drying problems. In large solid printed areas, the extra ink saturates the page and before it can properly dry, the image transfers or offsets onto the other pages as the web is rewound. However, reducing a significant amount of ink from a graphic with large printed areas may have the undesired effect of destroying the fine details in other parts of the image, where small fonts may be used. Furthermore, installing higher performance dryers adds extra expense and maintenance to the system. Alternatively, altering images at the press can be impractical in a high speed ink jet printing system, and would entail changes to the complex and highly optimized data station.
It would be desirable then to be able to programmatically alter the images being printed during data preparation or pre-press stages.
The present invention proposes a technique for processing 1-bit monochrome digital images and fonts to reduce ink usage on the printed page while maintaining image quality.
In accordance with one aspect of the present invention, a percentage of ink droplets is removed from the printed page while maintaining sharply defined edges in all areas of the image. An enhanced error diffusion technique is used to tone down the ink usage. Edge detection is used to ensure that fine details remain sharp and are not lost. These image processing operations are efficiently applied to the original source image by means of a bitmap mask.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.